The present invention relates to a solid-state electrolyte battery and cathode activating substance. The present invention relates more specifically to a solid-state electrolyte battery including a solid-state electrolyte not containing any organic electrolytic solution and to a cathode activating substance used for the same.
A lithium-ion secondary battery relying on doping or dedoping with lithium ions offers excellent energy density, thus finding application, for example, in mobile electronic devices. Among such lithium-ion secondary batteries, energetic research and development efforts have been under way on all-solid-state lithium-ion secondary batteries using, as an electrolyte, a solid-state electrolyte not containing any organic electrolytic solution.
Development efforts on a thin film lithium ion secondary battery, i.e., one form of such an all-solid-state lithium-ion secondary battery, are continuing at a brisk pace. This thin film lithium ion secondary battery includes a current collector, activating substance and electrolyte that are formed by thin films. Each of the thin films making up a thin film lithium ion secondary battery is formed by sputtering, vapor deposition or other film formation method (refer, for example, to Non-Patent Document 1).
In a thin film lithium secondary battery, an amorphous material such as LiPON or LiBON is used as a solid-state electrolyte. LiPON is obtained by substituting nitrogen to Li3PO4, and LiBON is obtained by substituting nitrogen to LixB2O4. The ionic conductivity of these materials is about 10−6 S/cm which is significantly lower than that of an ordinary liquid electrolyte of 10−2 S/cm. In a thin film lithium secondary battery, the film thickness of the solid-state electrolyte is small (e.g., about 1 μm). As a result, the distance traveled by Li is short. Therefore, the solid-state electrolyte made of the above amorphous material having a low ionic conductivity can offer performance equivalent to that of liquid electrolytes.
In a thin film lithium secondary battery, on the other hand, the cathode activating substance determines the rate of electrical conduction. It is common to use, as this cathode activating substance, LiCoO2, LiMn2O4, LiFePO4 or other lithium transition-metal oxide as with liquid-based lithium-ion secondary batteries. Further, in addition to the above, new lithium transition-metal oxides have been proposed for use as a cathode activating substance. For example, Patent Document 1 proposes crystalline LiCu1+xPO4 as a lithium transition-metal oxide for use as a cathode activating substance. These lithium transition-metal oxides (hereinafter referred to as the above lithium transition-metal oxides) are materials low in ionic conductivity and electron conductivity.
Of the above lithium transition-metal oxides, LiFePO4 is an environmentally-friendly material in that it is cheap and inexhaustible thanks to iron contained as its constituent element, thus gaining attention today. It should be noted, however, that LiFePO4 has a problem in that sufficient charge and discharge characteristics cannot be achieved because of its large internal resistance. Therefore, techniques have been proposed to reduce an impedance by coating the surface of LiFePO4 with carbon or lithium phosphate (refer, for example, to Non-Patent Document 2).
In a thin film lithium secondary battery, the thickness of the cathode activating substance layer is proportional to the battery capacity. In order to achieve high capacity, therefore, the cathode activating substance should be as thick as possible. In a thin film lithium secondary battery, however, increasing the thickness of the cathode activating substance layer made of a material low in ionic conductivity and electron conductivity (e.g., 10 μm or more) leads to a significantly large internal impedance.
Therefore, it is difficult to commercialize a high-capacity thin film lithium secondary battery having a thick cathode activating substance layer using any of the above lithium transition-metal oxides that are low in ionic conductivity and electron conductivity. In particular, LiFePO4 has poor electrical conduction. As a result, using LiFePO4 makes it impossible to increase the film thickness, thus making it difficult to commercialize a high-capacity thin film lithium secondary battery.
On the other hand, the above lithium transition-metal oxides are commonly used in a crystalline phase. Therefore, when a film of any of the above lithium transition-metal oxides is formed for a thin film lithium secondary battery, a crystalline phase is formed by heating the substrate during the film formation and post-annealing the substrate after the film formation.